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Ocean general circulation models

Ocean general circulation models routinely used in long climate simulations cannot afford to explicitly resolve some climatologically important small scale processes. These subgrid-scale processes include mesoscale and sub-mesoscale eddies as well as gravity current overflows. Therefore, they need to be parameterized to include their effects on the ocean circulation and climate. Much of my research focuses on the development, implementation, and climate impact assessment of new subgrid-scale parameterizations, using the ocean component of the NCAR Community Climate System Model (CCSM3).

During the past four years, these developments have been proceeding under the auspices of the U.S. CLIVAR Climate Process Teams (CPTs). These CPTs are highly collaborative projects each involving a team of theoreticians, observationalists, process modelers, and coupled climate modelers formed around specific issues or key uncertainties. The two ocean CPTs are on eddy - mixed layer interaction and gravity current entrainment. The primary goals are to expedite the incorporation of new parameterizations into ocean models and to assess their climate impacts.

The CPT on eddy mixed layer interaction has resulted in the implementation of a new near-surface eddy flux parameterization (in collaboration with R. Ferrari of MIT and J. C. McWilliams of UCLA) and a new prescription for the surface intensification and abyssal reduction of the tracer diffusivities (in collaboration with J. Marshall of MIT) in the ocean model. The former includes the effects of diabatic mesoscale fluxes within the surface layer. The experiments with the new parameterization show significant improvements compared to a control integration that tapers the effects of the eddies as the surface is approached. These improvements include the elimination of strong, near-surface, eddy-induced circulations and a better heat transport profile in the upper-ocean. Furthermore the need for any ad-hoc, near-surface taper functions is eliminated. The surface intensification and abyssal reduction of the tracer diffusivities are simply achieved using a stratification dependent vertical profile. These new solutions again compare more favorably with the available observations than those of the control which uses a constant value for both thickness and isopycnal diffusivities.

The CPT on gravity current entrainment has resulted in a new parameterized overflow scheme based on the marginal sea boundary condition of Price and Yang (1998, in Ocean Modeling and Parameterization, Kluwer Academic, 155-170.) This new parameterization represents exchanges through narrow straits (e.g., the Strait of Gibraltar) and channels (e.g., Faroe Bank Channel), associated entrainment, and intrusion of overflow product water into the open ocean (e.g., Atlantic). The model simulations in which this parameterization is used to represent the Mediterranean outflow through the Strait of Gibraltar show that the properties of the product water differ little from observed estimates and both uncoupled and coupled cases develop a Mediterranean salt tongue that spreads west and south from the Strait with a signature reminiscent of the observed hydrography. Without this parameterization, this salt tongue is absent in model solutions.

I am also interested in analysis and application of model simulations. One such example is a recent collaborative study with J. A. Kleypas of ISSE/NCAR and J. M. Lough of Australian Institute of Marine Science. Using data from both observations and an ensemble of CCSM3 Twentieth Century simulations, we have illustrated two important points regarding both the exposure and sensitivity of coral reefs to future bleaching events. First, exposure of coral reefs to increased sea surface temperatures (SSTs) has not been uniform over the last 50 years. SSTs greater than 29°C have warmed less than SSTs < 29°C (see figure). This is consistent with the notion that various negative feedbacks may act to slow the rate of warming in some tropical regions, i.e., the "ocean thermostat" mechanisms. The numbers of bleaching events that have occurred in regions where SSTs are already greater than 29°C appear to be less than in other regions. Second, coral reef sensitivity also varies geographically. The bleaching events that have occurred in these regions appear to have occurred at lower thresholds than the commonly used 1-2°C above the maxima. This study raises the issue that negative SST feedbacks may limit coral reef bleaching events in some areas and help provide desperately needed information regarding which reefs are likely to experience the least SST change, and consequently which are best suited for protection.

Figure caption: (A) Warming of average maximum SSTs between 1950-69 and 1987-2006 based on observations from HadISST data set; mean (large black dots) and standard deviations (vertical lines) are shown for 0.5°C intervals. Inset expands data for 29-31°C, with mean and standard deviation at 0.1°C intervals. Blue=Western Pacific Warm Pool, red=Red Sea; purple=Persian Gulf, green=Gulf of California, gray=all other locations. (B) As in A, based on CCSM3 four-member ensemble of 20th Century integrations. Note that 1980-99 range is used for the later years in B. The difference between SSTmax for 1950-69 and 1987-2006 based on observations shows that warming averages 0.2-0.4°C while SSTmax < 29.5°C; above 29.5°C the degree of warming drops off rapidly. Model data also agree with observations in that warming of maximum SSTs was less for regions with higher maximum SSTs. However, the modeled temperature where warming dropped off was higher than in observations and the reduction in warming at higher modeled SSTmax was also less pronounced than in the observations. (high resoultion figure)

For additional information, visit my homepage at: http://www.cgd.ucar.edu/oce/about/staff/gokhan/